Noisy thermostatic shower valve

I recently built a new home. I have a Riobel thermostatic shower valve and a Navien tankless with external recirculation. The shower valve regularly makes a buzzing noise when off - especially after a hot water faucet has been turned on and then off. I'm wondering if the recirculation system is 'backfeeding' through the thermostatic valve from hot to cold? Is that possible? If it is I'm assuming I need to install a check valve on the cold water supply line going into the valve?

Thank you.

Jack Morgan